1. Field of the Invention
The present invention relates generally to an ultrasonic flow or current meter and more particularly is directed to a very accurate ultrasonic flow or current meter.
2. Description of the Prior Art
A so-called sing-around method has been already proposed as a flow or current meter employing an ultrasonic wave. The prior art method will be now described with reference to FIGS. 1 and 2. In FIG. 1, reference numeral 1 designates a pipe through which a liquid such as water 2 to be measured flows in a direction shown by an arrow a, and 3 and 4 ultrasonic transducers each of which transmits and receives an ultrasonic signal or pulse (which will be hereinafter referred to as vibrators). The vibrators 3 and 4 are located on the pipe 1 at up- and down-stream sides with respect to the flow direction of the water 2. Ultrasonic sing-around systems A and B are formed of circuits or transceivers 5a and 5b, respectively.
The system A or transceiver 5a transmits a pulse Pa (refer to FIG. 2A) from its output terminal 6a at a time t.sub.1 shown in FIG. 2B. The pulse Pa arrives at an input terminal 7a of the transceiver 5a through the vibrator 3 - water 2 - vibrator 4, then is amplified in the transceiver 5a and again transmitted from the output terminal 6a of the transceiver 5a at a time t.sub.2 shown in FIG. 2B. This cycle will be repeated thereafter.
The system B is different from the system A in the direction of flow of a pulse Pb. That is, the pulse Pb transmitted from an output terminal 6b of the transceiver 5b at a time t'.sub.1 arrives at its input terminal 7b through the vibrator 4 - water 2 - vibrator 3, then is amplified in the transceiver 5b and again transmitted from its output terminal 6b at a time t'.sub.2 (refer to FIG. 2B). This cycle will be also repeated thereafter. Thus, reference symbol Ta shown in FIG. 2A represents one period of the pulse Pa, and Tb shown in FIG. 2B one period of the pulse Pb.
The periods Ta and Tb are equal to each other (Ta = Tb) when the water 2 is stopped, while when the water 2 flows or the flow velocity of the water 2 is not zero, the period Ta (forward direction) becomes shorter and the period Tb (backward direction) becomes longer and proportional to the velocity (that is, Ta.noteq.Tb). FIGS. 2A and 2B show the latter case.
If it is assumed that the flow velocity of the water 2 in the pipe 1 is taken as v (m/s), the velocity of the ultrasonic pulses Pa and Pb in the water 2 as c (m/s), the distance between the vibrators 3 and 4 along which the ultrasonic pulses propagate as L (m), and the angle between the flow direction a and the propagating direction of the ultrasonic pulse in the water 2 as .theta., the periods Ta and Tb can be expressed as follows: ##EQU1##
The repeating frequencies f.sub.a and f.sub.b of the respective pulses Pa and Pb can be expressed as follows: ##EQU2##
The difference .DELTA.f between the frequencies f.sub.a and f.sub.b is as follows: ##EQU3##
From the above expression it may be apparent that the frequency difference .DELTA.f is a function of only the flow velocity v and hence the flow velocity v can be obtained by measuring .DELTA.f. In this case, since the time interval during which the pulse propagates through the electric circuit system is very small as compared with that during which the pulse propagates through the water 2, the time interval in the electrical circuit is neglected.
With the prior art device shown in FIG. 1, since the periods Ta and Tb are different as may be seen from FIGS. 2A and 2B, even if the first pulses Pa and pb are emitted from the systems A and B at different times t.sub.1 and t'.sub.1, there will be a case when the pulses Pa and Pb are emitted at the same or approximately same time after the first emission of the pulses. In such a case, it becomes difficult to discriminate which of the pulses correspond to which of the systems A and B and hence there may be interference between the systems A and B.
Further, with the prior art the measurement is required to be made at a certain time interval, so that the response becomes slow.